I. Field of the Invention
The present invention is directed to a novel, essentially pollution-free method for the cationization of starch wherein an etherifying reagent and alkali catalyst are pre-mixed and reacted with starch using substantially dry reaction conditions. The resultant cationic starches find particular use in papermaking applications.
II. Brief Description of the Prior Art
Cationic starch derivatives are useful as additives in a variety of industrial applications. For example, cationic starch derivatives are often used in the paper manufacturing industry and other industries as flocculating agents for colloidal solutions. Moreover, they are particularly useful as beater and/or headbox additives in the manufacture of paper wherein their inherent cationic charge significantly improves the retention of inorganic pigments and starch by the cellulose pulp without a loss of strength, and when used without pigments it improves the strength of the finished paper. Many methods are known for the production of cationic starches using a variety of reagents under both aqueous and "dry" reaction conditions. Major disadvantages to the available aqueous cationization processes include employing large quantities of water, long reaction times, and inferior reaction efficiencies. Moreover, the necessity for recycling or otherwise disposing of large quantities of processing waters presents serious ecological problems. In current research, emphasis has therefore been placed on providing efficient commercially and economical methods for the cationization of starch using dry (substantially dry) reaction conditions.
In particular, U.S. Pat. No. 3,346,563 teaches the preparation of quaternary ammonium ethers of starch using, as a reagent, a specifically prepared quaternary halohydrin salt. In accordance with the dry reaction disclosed therein, ungelatinized starch is tumbled and heated with crystalline N-(3-chloro-2-hydroxypropyl)trimethylammonium chloride and solid alkali catalyst. The prolonged tumbling, or blending, requirement renders this process commercially unacceptable and often results in localized, non-uniform cationization of the starch.
Subsequently issued U.S. Pat. No. 3,448,101 teaches preparation of cationic starch ethers by blending granular starch with a non-volatile epoxide containing a tertiary or quaternary ammonium salt group and heating the blend in an essentially dry state and the absence of catalyst at a temperature of 200.degree.-300.degree. F. (93.degree.-149.degree. C.). Examples presented herein illustrate the relative inferior reaction efficiency achieved in following the teachings of the above-mentioned U.S. Pat. No. 3,448,101.
In contrast, U.S. Pat. No. 4,127,563 describes a dry reaction of starch with a similar reagent in the halohydrin form using an alkali catalyst at a pH of 5-9 and a temperature of 25.degree.-100.degree. C. Attempts to prepare cationic starches in accordance with the teaching of U.S. Pat. No. 4,127,563 have, likewise, resulted in lower reaction efficiencies, as will be shown hereinbelow. Such lower reaction efficiencies are not generally acceptable in commercial practice and are particularly objectionable for dry reactions where any residual reagent and by-products would normally remain in the product.
It is therefore an object of the present invention to provide a commercially efficient, pollution-free method for the preparation of cationic starch products using substantially dry reaction conditions.